In the process known as electrocoating or electrodeposition painting, paint solids (pigment and resin) are maintained in solution or as a colloidal suspension by a counter-ion that functions as the solubilizer. In most cases, the paint solids are positively charged, or cationic, and the solubilizer counter-ion is an anion (called cathodic electrocoat). However, in some systems the paint solids are anodic and the counter-ion solubilizer is the cation (called anodic electrocoat). The design and principles disclosed here can apply to either. The examples and design specifics are therefore expressed in terms of the “object electrode”, which is the item or “ware” to be painted or the electrode opposite in polarity to the bulk of the paint pigment and resin, and the “counter electrode”, which is the electrode of polarity opposite to the object electrode.
In the conventional cathodic electrocoat painting process, the ware to be painted is the electrode and is physically submerged in a container, or so-called “bath”, of the paint solution along with one or more counter electrodes that in this particular instance serve as anodes. An analogous cell and process for anodic electrocoat paint can be made by reversing cathode and anode positions.
An electrical field is applied between the object electrode, and the counter electrode, by means of a DC power supply. Paint solids are thereby attracted to and deposited on the ware. In most electrocoat painting systems, the counter electrode is part of an electrodialysis cell in which the counter electrode is inside an enclosure and at least a portion of this enclosure is provided by an ion-exchange membrane material. Electrolyte is circulated inside the enclosure between the counter electrode and ion-exchange membrane and the solubilizer counter-ion is drawn through the membrane into the electrolyte by the electrical field. Electrolyte is periodically withdrawn from this circulating loop and disposed of. In this manner, the composition of the paint is maintained in chemical balance. If, by contrast, only the paint solids are removed in the process and the solubilizer is allowed to accumulate in the solution, the solubilizer-to-paint solids ratio will increase and the paint will become unstable.
A second item of process equipment that is essential to the electrocoating process, in addition to the above described electrodialysis cells, is the ultrafilter. The ultrafilter performs two important functions. First, it produces ultrafiltrate, or permeate, which is used to wash entrained paint off of the ware after the ware is removed from the paint bath and return this paint to the paint bath. The recovery of this paint is essential to the economics and environmental acceptability of electrocoating. If the excess paint were washed from the ware with a solution that had not been extracted from the paint bath, this solution could not be returned to the bath because the bath must be maintained as a closed system. In addition to paint recovery, the ultrafilter plays a role in maintaining paint bath chemistry in that the ultrafiltrate can also be purged from the system to remove contaminants that might otherwise accumulate in the paint bath.
Ultrafiltration of electrocoat paint and removal of paint solubilizer by electrodialysis are well-known processes in the prior art. Also established in the prior art is the idea of removing the solubilizer by electrodialysis of the ultrafilter permeate rather than via cells placed directly in the paint bath, for example U.S. Pat. No. 4,581,111 and U.S. Pat. No. 3,663,405 both to Christenson et al. However, this “external electrodialysis” system has never been commercially successful.
Removal of the solubilizer by electrodialysis of permeate instead of direct electrodialysis in the paint bath is commercially attractive, however, for several reasons:                1. Electrodialysis cells must be periodically inspected, maintained, and occasionally replaced. This is difficult to accomplish with in situ cells because the paint bath is typically inside an enclosure for safety reasons. The paint is also unpleasant to work with because it contains hazardous chemicals and is difficult to remove from skin or clothing. By contrast, external electrodialysis of permeate takes place outside of the paint bath enclosure and the permeate, which is substantially water and trace amounts of solvents and solubilizer, is significantly more pleasant to work with than the paint itself. External cells are also accessible while the paint line is in operation, so that maintenance can be performed at the convenience of the user.        2. Direct, or in situ, electrodialysis is accomplished by the same power source that causes the deposition of the paint and is therefore generally in proportion to the painting rate. At first glance, this would appear to be advantageous; however, it has disadvantages. First, the direct process naturally removes more solubilizer than is required to maintain the bath chemistry. It is therefore necessary to add solubilizer back to the system. The user must therefore purchase and handle this solubilizer and pay for the waste treatment on the excess solubilizer that is removed from the system. On the other hand, it is not unusual for the cells to leak of overflow. This creates an excess of solubilizer in the paint bath which can only be removed as part of the painting process. This means that returning the paint bath to the desired composition after such an excursion may take many days or even weeks. For these reasons, it is often difficult to control precisely the paint bath chemistry with situ or direct electrodialysis. By contrast, since external electrodialysis operates independently of the painting process, more precise control of paint bath chemistry is possible and it is possible to recover from major excursions even during non-production periods.        3. Some electrocoating operations with very low or intermittent production have difficulty removing enough solubilizer by in situ electrodialysis because solubilizer removal is only taking place concurrently with painting. However, since the ultrafilter operates continuously and independent of the painting process, and the permeate is therefore available continuously, with external electrodialysis it is possible to remove as much or as little solubilizer as needed at any given time to maintain paint bath chemistry.        4. Some other electrocoating operations have a very high ware area in comparison to the size of the paint bath (for example, the painting of heat exchangers) and it is physically difficult to place enough electrodialysis cells in the paint bath to accomplish the desired solubilizer removal. Again by contrast, these applications can be easily satisfied by external electrodialysis.        
In spite of these obvious advantages and numerous commercial attempts, electrodialysis of the permeate has not been commercially successful, primarily for two reasons. First, with in situ electrodialysis, the ware itself functions as the object electrode. For permeate electrodialysis, a separate, sacrificial object electrode must be provided to serve as the opposing electrode in the cell. Even though the ultrafilter permeate is substantially water, it also contains some paint pigment and low molecular weight resin. This pigment and resin tend to become coated on the sacrificial object electrode and accumulates over time. Eventually this coating increases the electrical resistance of the cell and the object electrode must be cleaned or replaced.
Second, the electrodialysis cells that have been applied to ultrafilter permeate have been of the so-called plate and frame design, or variations thereof, in which the cell consists of alternating flat plate object and counter electrodes with ion exchange membranes and flow channel spacers sandwiched in between as shown in FIG. 1 for plate and frame cell 10. Electrolyte 12 and permeate 14 are circulated in directions indicated by arrows through the respective spacer channels defined by anodes 16 and 18, cathode 20 and ion exchange membranes 22 and 24. Solubilizer ions 26 and 28 pass through respective membranes 22 and 24 while paint particles 30 and 32 attach to cathode 20. This classic cell design has the advantage of minimizing the voltage required to operate the cell. However, it has the disadvantage of being difficult to disassemble for servicing in the event of cathode fouling as described above, or anode dissolution, membrane leaks, etc. In the case of electrodialysis of electrocoat paint ultrafilter permeate, both object electrode fouling and anode dissolution are common. Accordingly, the plate and frame design for electrodialysis cells is not commercially attractive.